Method for improving the wear performance of ceramic-polyethylene or ceramic-ceramic articulation couples utilized in orthopaedic joint prostheses

ABSTRACT

Methods for improving the wear performance of silicon nitride and/or other ceramic materials, particularly to make them more suitable for use in manufacturing biomedical implants.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/655,457, filed Apr. 10, 2018, the contents of which are entirelyincorporated by reference herein.

FIELD

The present disclosure generally relates to methods for producingsilicon oxynitride materials that have improved polyethylene wearperformance.

BACKGROUND

Orthopaedic reconstructive surgeries, including total hip (THA), totalknee (TKA), or total shoulder (TSA) arthroplasty, are proven proceduresfor treatment of various end-stage degenerative osteoarthropathyconditions. These therapies involve the replacement of native biologicalarticulation tissues with abiotic biomaterials. Typical THA prostheticdevices include mobile metallic or ceramic heads articulating againststationary polyethylene counterfaces (MoP or CoP, respectively). Othervariations include ceramic-on-ceramic (CoC) devices. While the longevityof these prostheses are reasonable (i.e., 10-15 years), their failure isgenerally associated with excessive polyethylene wear, ceramic wear, orcomponent damage which results in aseptic loosening, osteolysis, and/orosteomyelitis. Revision surgery (an unwanted and expensive secondaryprocedure for both the surgeon and hospital) is then required to replacethe worn components, often resulting in poorer ambulatory function withadded comorbidities for the patient. Therefore, there is a need formaterials that have increased wear performance that can be used inprostheses.

It is with these observations in mind, among others, that variousaspects of the present disclosure were conceived and developed.

SUMMARY OF THE INVENTION

One aspect of the present disclosure encompasses a silicon oxynitridematerial, wherein the silicon oxynitride material has improved wearperformance. The silicon oxynitride material is prepared by a processcomprising forming a silicon nitride material block and oxidizing thesilicon nitride material block.

Other aspects and features of the invention will be in part apparent andin part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present patent or application file contains at least one drawingexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

FIG. 1A and FIG. 1B depict schematic diagrams for the surface chemistryof Si₃N₄ ceramics: (FIG. 1A) prior to hydrothermal oxidation; and, (FIG.1B) after hydrothermal oxidation. Note the reduced concentration ofamines and increased concentration of silanols and silica (SiO₂) bondingin the hydrothermally oxidized surface.

FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D, FIG. 2E, FIG. 2F, FIG. 2G, and FIG.2H depict graphs showing x-ray photoelectron spectroscopy (XPS) resultsfor hydrothermally-treated silicon nitride surfaces after 0 (FIG. 2A andFIG. 2E), 24 (FIG. 2B and FIG. 2F), 48 (FIG. 2C and FIG. 2G), and 72(FIG. 2D and FIG. 2H) hours of exposure to the hydrothermal oxidationprocess. FIG. 2E, FIG. 2F, FIG. 2G, and FIG. 2H show deconvolution ofthe O1s band. FIG. 2E, FIG. 2F, FIG. 2G, and FIG. 2H show deconvolutionof the Si₂P band.

FIG. 3A and FIG. 3B depict graphs showing statistical analysis andsignificance of XPS data.

FIG. 4A and FIG. 48 depict microstructural photographs of polished Si₃N₄surfaces before (i.e., pristine, FIG. 4A) and after (i.e., oxidized,FIG. 48) a 72 hour hydrothermal treatment demonstrating that thetreatment fills in the pores or voids in the ceramic surface.

FIG. 5 depicts a graph illustrating polyethylene wear results from astandard hip simulator study comparing MC²® Si₃N₄ to BIOLOX®delta (ZTA).

FIG. 6A and FIG. 6B depict graphs showing Raman spectroscopymeasurements of crystallinity and oxidation for vitamin E dopedpolyethylene liners articulating against either Si₃N₄ or ZTA femoralheads for both non-wear-(NWZ) and main-wear-zones (MWZ): (FIG. 6A) atthe sliding surface, z=0 μm; and (FIG. 6B) at a depth of z=200 μm.

FIG. 7 depicts representative images of room-temperature evolution of pHsurrounding an as-sintered (polished) Si₃N₄ sample as a function of timein an acidic gel. The buffering ability of Si₃N₄ gradually increases pHin ever wider zones of the surrounding acidic gel. The average pH of theunperturbed gel=4.5.

FIG. 8A is a schematic diagram of the static contact test in anautoclave used for UHMWPE/ceramic couples. FIG. 8B illustrates thefrictional swing test used for UHMWPE/ceramic couples. FIG. 8Cillustrates a hip simulator wear test used for UHMWPE/ceramic couples.

FIG. 9A shows XPS spectra and their deconvolution into sub-bands forAl2p in Al₂O₃(BIOLOX®forte) as received. FIG. 9B shows XPS spectra andtheir deconvolution into sub-bands for Al2p in ZTA (BIOLOX®delta) asreceived.

FIG. 9C shows XPS spectra and their deconvolution into sub-bands for andSi2p in Si₃N₄(MC²®) as received. FIG. 9D shows XPS spectra and theirdeconvolution into sub-bands for Al2p in Al₂O₃ (BIOLOX®forte) after 24 hadiabatic exposure in autoclave at 121° C. FIG. 9E shows XPS spectra andtheir deconvolution into sub-bands for Al2p in ZTA (BIOLOX®delta) after24 h adiabatic exposure in autoclave at 121° C. FIG. 9F shows XPSspectra and their deconvolution into sub-bands for and Si2p inSi₃N₄(MC²®) after 24 h adiabatic exposure in autoclave at 121° C.

FIG. 10A shows XPS analyses as a function of autoclaving time formonolithic Al₂O₃(Al2p) ceramic heads. FIG. 10B shows XPS analyses as afunction of autoclaving time for ZTA (Al2p) ceramic heads. FIG. 10Cshows XPS analyses as a function of autoclaving time for Si₃N₄(O1s)ceramic heads. FIG. 10D shows XPS analyses as a function of autoclavingtime for Si₃N₄(Si2p) ceramic heads.

FIG. 11A shows CL analyses as a function of autoclaving time on spectralevolution in monolithic Al₂O₃. FIG. 11B shows CL analyses as a functionof autoclaving time on spectral evolution in ZTA composite. FIG. 11Cplots the intensity of the CL emissions from oxygen vacancies versusautoclaving time for two types of oxide heads. FIG. 11D shows acomparison between XPS and CL data for two types of oxide heads.

FIG. 12A shows variations of crystallinity and oxidation indices asdetected by vibrational spectroscopy for X3 UHMWPE liners staticallycoupled to oxide and non-oxide ceramic heads for 24 h in an autoclave.FIG. 12B shows XPS analyses of the same liners in FIG. 12A.

FIG. 13A shows XPS (Al2p) analyses of ZTA femoral heads beforefrictional swing testing against X3 UHMWPE liners for 5×10⁵ cycles at 1Hz.

FIG. 13B shows XPS (Al2p) analyses of ZTA femoral heads after frictionalswing testing against X3 UHMWPE liners for 5×10⁵ cycles at 1 Hz. FIG.13C shows quantitative bond fractions are given in (Al2p). FIG. 13Dshows quantitative bond fractions are given in (O1s and Zr3d).

FIG. 14A shows XPS (N1s) analyses of Si₃N₄ femoral heads beforefrictional swing testing against X3 UHMWPE liners for 5×10⁵ cycles at 1Hz.

FIG. 14B shows XPS (N1s) analyses of Si₃N₄ femoral heads afterfrictional swing testing against X3 UHMWPE liners for 5×10⁵ cycles at 1Hz. FIG. 14C shows quantitative bond fractions are given in (N1s). FIG.14D shows quantitative bond fractions are given in (Si2p and O1s).

FIG. 15A shows crystallinity at the surface of X3 UHMWPE liners coupledto Si₃N₄ and ZTA as a function of the number of cycles, n_(c), offrictional swing testing. FIG. 15B shows oxidation at the surface of X3UHMWPE liners coupled to Si₃N₄ and ZTA as a function of the number ofcycles, n_(c), of frictional swing testing.

FIG. 16A shows scanning laser microscopy images of pristine and MWZ wornsurfaces (after 5×10⁵ swing cycles) of X3™ UHMWPE liners coupled to ZTAand Si₃N₄ femoral heads. FIG. 16B shows results of XPS analyses in NWZand MWZ of the same liners.

FIG. 17A shows crystallinity and oxidation variations observed at thesurfaces of vitamin E-doped UHMWPE liners coupled to Si₃N₄ and ZTAfemoral heads after being subjected to a 5×10⁶ cycles in a standard hipsimulator test. FIG. 17B shows crystallinity and oxidation variationsobserved at 200 μm in the depth of vitamin E-doped UHMWPE liners coupledto Si₃N₄ and ZTA femoral heads after being subjected to a 5×10⁶ cyclesin a standard hip simulator test.

FIG. 18A shows a long-term in vivo exposed monolithic Al₂O₃ femoralhead. FIG. 18B shows the microstructure in the MWZ of a long-term invivo exposed monolithic Al₂O₃ femoral head. FIG. 18C shows itsmicrostructure in the NWZ. FIG. 18D shows its CL oxygen vacancyemissions compared to that of a pristine Al₂O₃ sample. FIG. 18E showsits deconvoluted average XPS (Al2p) spectrum.

FIG. 19A shows a short-term in vivo exposed ZTA composite femoral head.FIG. 19B shows the microstructure of a short-term in vivo exposed ZTAcomposite femoral head. FIG. 19C shows its microstructure in the NWZ.FIG. 19D shows its CL oxygen vacancy emission compared to that of apristine ZTA sample. FIG. 19E shows its deconvoluted average XPS (Al2p)spectrum.

Corresponding reference characters indicate corresponding elements amongthe view of the drawings. The headings used in the figures do not limitthe scope of the claims.

DETAILED DESCRIPTION

It should be understood from the foregoing that, while particularembodiments have been illustrated and described, various modificationscan be made thereto without departing from the spirit and scope of theinvention as will be apparent to those skilled in the art. Such changesand modifications are within the scope and teachings of this inventionas defined in the claims appended hereto.

Several definitions that apply throughout the above disclosure will nowbe presented. As used herein, “improved wear performance” means animprovement in the longevity of the material or device over existing THAprosthetic devices. For example, “improved wear performance” means thematerial and/or device has a longevity of greater than 10-15 years afterbeing implanted in a patient. The terms “comprising,” “including” and“having” are used interchangeably in this disclosure. The terms“comprising,” “including” and “having” mean to include, but notnecessarily be limited to the things so described.

There are crucial physical chemistry characteristics of biomaterialsurfaces that directly affect their long-term performance as artificialjoints. Non-oxide bioceramics, such as silicon nitride, may possessfavorable surface chemistry that naturally protects apolyethylene-sliding counter-surface from oxidation. A key concept inestablishing this favorable chemistry is the control of the oxygenactivity at the bioceramic surface during tribochemical loading in theotherwise anaerobic body environment.

Ceramic oxides, which are comprised of metal and oxygen elements,exhibit significant affinity for water because of highly synergichydrogen bonding at the liquid/solid interface. In the case of alumina(Al₂O₃), a peculiar near-surface electronic state provides multipleH-bonding, which results in complete wetting—a positive phenomenon inhip-joint tribology. However, this same peculiarity leads to complexpatterns of surface hydroxylation and dehydroxylation in thermally- orfrictionally-activated environments. Hydroxylation and dehydroxylationare key events in rationalizing surface charge issues; they playimportant roles in frictional interactions, although their precisemicroscopic mechanisms are presently unknown. The incorporation of waterinto the Al₂O₃ crystal structure results in the formation of aluminumhydroxide. Dissolution of alumina via amphoteric ionization reactionsfrees oxygen and forms oxygen vacancies within the alumina lattice. Thesubsequent release of soluble Al species as hydrolysis products isdependent on both pH and temperature. Conversely, hydrothermalinteractions between non-oxide ceramics and their environment is mainlydriven by oxidation of their cation elements. In the case of siliconnitride (Si₃N₄), surface reactions start with homolytic cleavage of thecovalent bond between silicon and nitrogen, followed by oxidation of thesilicon sites, and the release of nitrogen as ammonia. During frictionalloading in an aqueous environment, a layer of insoluble tribo-products(i.e., hydrated silicon oxides) forms at the solid surface.Collectively, they act as a lubricant in frictional sliding by forming aprotective film. The advantage of this hydrated layer in reducingfriction is similar to that of the hydrated layer in Al₂O₃. However,this is where the similarity ends. Oxygen is attracted to the non-oxideceramic's surface (at Si sites) rather than being released (as is thecase for Al₂O₃), while nitrogen reacts with hydrogen to form volatileammonia. Moreover, the amphoteric silica layer formed at the surface ofSi₃N₄ acts as an Arrhenius acid with water being the correspondingArrhenius base. Also, the surface charge of Si₃N₄ depends on the pH ofthe environment; its isoelectric point resides at extremely acidicvalues (pH=1.2-4). Conversely, Al₂O₃ has a point of zero charge atrelatively high alkaline values (pH=8-8.5). The silica layer thatdevelops at the H₂O-chemisorbed surface of Si₃N₄ can easily dissolvebecause it is considerably more acidic than water, (i.e., its solubilityis ˜100 times that of Al₂O₃), but oxygen is tightly bound asorthosilicic acid chains. In essence, water adsorption at the surface ofceramics acts as a solvent for oxides and as an oxidant for non-oxides.In both cases, the final products of these aqueous surface reactions arehydrated species (i.e., aluminum hydroxides and orthosilicic acid forAl₂O₃ and Si₃N₄, respectively). Both act as lubricants to reducefriction during tribological sliding. While this common characteristicmakes both oxide and non-oxide ceramics suitable as low-frictionartificial joint materials, they substantially differ in the directionof oxygen flow across the tribolayer. Specifically, oxygen moves awayfrom the Al₂O₃ surface and moves towards the Si₃N₄ surface. Thisdifference is crucial when the sliding counterpart in the artificialjoint is polyethylene.

The oxygen released from various oxide ceramics' surfaces may lead tothe oxidation of advanced polyethylene liners. Silicon nitride withoxidized surfaces (silicon oxynitride) may have a much lower impact onpolyethylene liner oxidation and may provide an “ionic protective”effect. Silicon nitride ceramics in femoral heads may delay oxidation ofpolyethylene liners. Therefore the ultimate lifetime of artificialjoints may be improved by the use of silicon nitride femoral heads withan oxidized surface.

(I) Silicon Oxynitride Materials

An aspect of the present disclosure encompasses silicon oxynitridematerials that have improved wear performance or characteristics. Ingeneral, the silicon oxynitride materials may be formed by oxidizing thesurface of a silicon nitride material.

The silicon oxynitride material may form a biomedical implant or part ofa biomedical implant in various embodiments. In preferred embodiments,silicon oxynitride material implants, may therefore be provided thatmay, in some embodiments, be treated so as to improve upon their wearcharacteristics, water wettability, and/or other desirablecharacteristics.

In other embodiments, the silicon oxynitride material may comprise anunfinished piece of material that will ultimately be shaped, machined,or otherwise formed into a suitable shape and/or configuration to serveas one of the above-referenced finished biomedical implants. In somesuch embodiments, the unfinished piece may require one or moreadditional processing steps before it can be considered completed andready for implantation. For example, in some embodiments, the biomedicalimplant may comprise only a part or portion of what will eventuallybecome a finished biomedical implant. In one embodiment, the biomedicalimplant is an articulation component. Examples of articulationcomponents may be, without limit, femoral heads, femoral condyles,acetabular cups/liners, etc. In an exemplary embodiment, thearticulation component may be a femoral head.

As still another alternative, the silicon oxynitride materials disclosedherein may be used as a filler or otherwise incorporated into othermaterials, such as glasses, metals, ceramics, polymers, and the like.For example, in some embodiments, one or more of the ceramic materialsdisclosed herein may be used as a filler in a polymeric material.Conversely, the ceramic material disclosed herein could be used as aporous matrix to incorporate polymeric materials, glasses, or metals.

In alternative embodiments and implementations, the surface chemistry ofa silicon oxynitride material may be altered to improve the wearperformance characteristics of such implants. In some suchimplementations, the chemistry of the surface of a monolithic device orcoating on a silicon oxynitride implant, silicon oxynitride coatedimplant, or other implantable biomedical implant, may be modified toimprove wear performance characteristics. These methods for altering thesurface chemistry may be employed as an alternative to, or in additionto, other methods described herein, such as methods for changing thesurface roughness of an implant and/or applying a suitable coating to abiomedical implant. These methods for altering the surface chemistry mayalso be accomplished in several ways, as further described below.

(II) Methods of Preparing Silicon Oxynitride Materials

Another aspect of the present disclosure encompasses a process forpreparing a silicon oxynitride material comprising forming a siliconnitride material block and oxidizing the silicon nitride material block.The method may produce a silicon oxynitride implant with improved wearperformance.

Each of the steps of the method is detailed below.

(a) Silicon Nitride

In general, the silicon nitride may be made out of silicon nitrideceramic or doped silicon nitride ceramic substrate. Alternatively, suchembodiments may comprise a silicon nitride or doped silicon nitridecoating on a substrate of a different material. In other embodiments, animplant and the coating may be made up of a silicon nitride material. Instill other embodiments, one or more portions or regions of an implantmay include a silicon nitride material and/or a silicon nitride coating,and other portions or regions may include other biomedical materials.

Silicon nitride ceramics have tremendous flexural strength and fracturetoughness. In some embodiments, such ceramics have been found to have aflexural strength greater than about 700 Mega-Pascal (MPa). Indeed, insome embodiments, the flexural strength of such ceramics have beenmeasured at greater than about 800 MPa, greater than about 900 MPa, orabout 1,000 MPa. The fracture toughness of silicon nitride ceramics insome embodiments exceeds about 7 Mega-Pascal root meter (MPa·m^(1/2)).)Indeed, the fracture toughness of such materials in some embodiments isabout 7-10 MPa·m^(1/2).

Examples of suitable silicon nitride materials are described in, forexample, U.S. Pat. No. 6,881,229, titled “Metal-Ceramic CompositeArticulation,” which is incorporated by reference herein. In someembodiments, dopants such as alumina (Al₂O₃), yttria (Y₂O₃), magnesiumoxide (MgO), and strontium oxide (SrO), can be processed to form a dopedcomposition of silicon nitride. In embodiments comprising a dopedsilicon nitride or another similar ceramic material, the dopant amountmay be optimized to achieve the highest density, mechanical, and/orantibacterial properties. In further embodiments, the biocompatibleceramic may have a flexural strength greater than about 900 MPa, and atoughness greater than about 9 MPa·m^(1/2). Flexural strength can bemeasured on standard 3-point bend specimens per American Society forTesting of Metals (ASTM) protocol method C-1161, and fracture toughnesscan be measured using single edge notched beam specimens per ASTMprotocol method E399. In some embodiments, powders of silicon nitridemay be used to form the ceramic implants, either alone or in combinationwith one or more of the dopants referenced above.

Other examples of suitable silicon nitride materials are described inU.S. Pat. No. 7,666,229 titled “Ceramic-Ceramic Articulation SurfaceImplants,” which is hereby incorporated by reference. Still otherexamples of suitable silicon nitride materials are described in U.S.Pat. No. 7,695,521 titled “Hip Prosthesis with Monoblock CeramicAcetabular Cup,” which is also hereby incorporated by reference.

(i) Method of Preparing the Silicon Nitride Material Block

In an embodiment, preparing the silicon nitride material block maycomprise preparing a slurry, where the slurry may comprise silicon,oxygen, and nitrogen, and may further comprise at least one of yttriumoxide and aluminum oxide.

The slurry may be milled to break up soft agglomerates and facilitateconstituent mixing. In general, the slurry may be milled usingtechniques know to those of skill in the art. In an exemplaryembodiment, the slurry is ball milled. Additionally, those of skill inthe art would be able to determine the appropriate media, media size,and duration for the milling process.

The slurry may be dried to obtain a dried slurry, after which the driedslurry may be formed into a number of different shapes for femoralheads, articulation components, or the like. In general, the slurry maybe dried using techniques known to those of skill in the art. In anexemplary embodiment, the slurry is dried using spray drying.

In general, the silicon nitride material block may be applied tobiomedical components or formed or shaped into a biomedical implant. Inone example, the silicon nitride material block may be formed or shapedinto an articulation component. Examples of articulation components maybe, without limit, femoral heads, femoral condyles, acetabular cups,etc. In an exemplary embodiment, the articulation component may be afemoral head.

In other embodiments, the silicon nitride material block may be appliedto any number and type of biomedical components including, withoutlimit, spinal cages, orthopedic screws, plates, wires, and otherfixation devices, articulation devices in the spine, hip, knee,shoulder, ankle and phalanges, catheters, artificial blood vessels andshunts, implants for facial or other reconstructive plastic surgery,middle ear implants, dental devices, and the like. In an example, thesilicon nitride material block may be applied to a prosthetic joint,such as a femoral head of a THA prosthesis.

Applying the silicon nitride material block to biomedical components maybe performed by methods readily known by those of skill in the art.

The forming or shaping the silicon nitride material block may beperformed by methods readily known by those of skill in the art. In anexemplary embodiment, the directed slurry may be consolidated usinguniaxial or isostatic compacting equipment to form appropriate shapes.These shapes may then be subsequently machined to pre-fired dimensionsusing conventions computer-numerically-controlled (CNC) turning ormilling machinery. In some embodiments, the silicon nitride materialblock may be formed into any number and type of biomedical componentsincluding, without limit, spinal cages, orthopedic screws, plates,wires, and other fixation devices, articulation devices in the spine,hip, knee, shoulder, ankle and phalanges, catheters, artificial bloodvessels and shunts, implants for facial or other reconstructive plasticsurgery, middle ear implants, dental devices, and the like. In anexample, the silicon nitride material block may be applied to aprosthetic joint, such as a femoral head of a THA prosthesis.

The appropriately shaped liners or components may then be subjected to aseries of heat-treatment operations including, without limit, bisquefiring, sintering, and hot-isostatic pressuring.

The heat-treated liners or components may then be subjected to diamondgrinding and polishing to achieve the final size and surface finish.

(b) Oxidation Methods

The surface of the silicon nitride material may be oxidized by thermal,hydrothermal, or chemical oxidation. In general, the oxidation methodsdescried herein convert some of the Si₃N₄ to SiO₂ on the surface of thematerials.

(i) Thermal Oxidation

In general, the surface of the silicon nitride material may be oxidizedusing thermal oxidation. The thermal oxidation process may be conductedusing means known to those of skill in the art.

In general, the thermal oxidation process may be conducted at atemperature of up to about 1100° C. In preferred embodiments, thethermal oxidation process may be conducted a temperature ranging fromabout 800 to about 1100° C.

The thermal oxidation process may be conducted for a duration rangingfrom about 5 hours to about 20 hours. In some embodiments, the thermaloxidation process may be conducted for about 5, about 6, about 7, about8, about 9, about 10, about 11, about 12, about 13, about 14, about 15,about 16, about 17, about 18, about 19, or about 20 hours.

(ii) Hydrothermal Oxidation

In general, the surface of the silicon nitride material may be oxidizedusing hydrothermal oxidation. The hydrothermal oxidation process may beconducted using means known to those of skill in the art. In anexemplary embodiment, the hydrothermal oxidation may be performed in asteam autoclave. The effects of hydrothermal oxidation process on thesurface chemistry of Si₃N₄ ceramics is illustrated in FIG. 1A (prior tohydrothermal oxidation) and FIG. 1B (after hydrothermal oxidation).

In general, the hydrothermal oxidation process may be conducted atpressures ranging from about 1 atmosphere to about 250 atmospheres. Infurther, embodiments, the hydrothermal oxidation process may beconducted at a pressure of about 1, about 2, about 3, about 4 about 5,about 6, about 7, about 8, about 9, about 10, about 15, about 20, about25, about 30, about 35, about 40, about 45, about 50, about 55, about60, about 65, about 70, about 75, about 80, about 85, about 90, about95, about 100, about 105, about 110, about 115, about 120, about 125,about 130, about 135, about 140, about 145, about 150, about 155, about160, about 165, about 170, about 175, about 180, about 185, about 190,about 195, about 200, about 205, about 210, about 215, about 220, about225, about 230, about 235, about 240, about 245, or about 250atmospheres. In an exemplary embodiment, the hydrothermal oxidationprocess may be conducted at a pressure of about 2 atmospheres.

The hydrothermal oxidation process may be conducted for a durationranging from about 50 to about 200 hours. In some embodiments, thehydrothermal oxidation may be conducted for about 50, about 55, about60, about 65, about 70, about 75, about 80, about 85, about 90, about95, about 100, about 105, about 110, about 115, about 120, about 125,about 130, about 135, about 140, about 145, about 150, about 155, about160, about 165, about 170, about 175, about 180, about 185, about 190,about 195, or about 200 hours. In an exemplary embodiment, thehydrothermal oxidation process may be conducted for a duration rangingfrom about 70 to about 150 hours.

The hydrothermal oxidation process may be conducted a temperatureranging from about 100° C. to about 150° C. In some embodiments, thehydrothermal oxidation may be conducted at about 100, about 105, about110, about 115, about 120, about 125, about 130, about 135, about 140,about 145, or about 150° C. In preferred embodiments, the hydrothermaloxidation may be conducted from about 120° C. to about 135° C. Infurther embodiments, the hydrothermal oxidation may be conducted atabout 120, about 121, about 122, about 123, about 124, about 125, about126, about 127, about 128, about 129, about 130, about 131, about 132,about 133, about 134, or about 135° C.

(iii) Chemical Oxidation

In general, the surface of the silicon nitride material may be oxidizedusing chemical oxidation. The chemical oxidation process may beconducted using means know to those of skill in the art.

The chemical oxidation process may be conducted by exposing the siliconnitride material to caustic solutions. The caustic solutions mayinclude, without limit, sodium hydroxide, ammonium hydroxide, calciumhydroxide, etc. and combinations thereof.

EXAMPLES

The following examples are included to demonstrate various embodimentsof the present disclosure. It should be appreciated by those of skill inthe art that the techniques disclosed in the examples that followrepresent techniques discovered by the inventors to function well in thepractice of the invention, and thus can be considered to constitutepreferred modes for its practice. However, those of skill in the artshould, in light of the present disclosure, appreciate that many changescan be made in the specific embodiments which are disclosed and stillobtain a like or similar result without departing from the spirit andscope of the invention.

Example 1: Preparation of Biocompatible Silicon Nitride CeramicComponents

α-Si₃N₄ (90 wt. %), yttrium oxide (Y₂O₃, 6 wt. %), and aluminum oxide(Al₂O₃, 4 wt. %) raw powders were admixed in water, milled, and spraydried. The spray dried powders were then consolidated using uniaxial orisostatic compacting equipment (up to 310 MPa) to form appropriateshapes, i.e., femoral heads and mechanical test-bars. These componentswere subsequently machined to pre-fired dimensions using conventionalcomputer-numerically-controlled (CNC) turning or milling machinery. Theywere then subjected to a series of heat-treatment operations includingbisque firing, sintering, and hot-isostatic pressing at temperatures upto 1700° C. The firing steps eliminated carbonaceous compounds andwater, reacted the constituent raw materials, and densified the ceramicto near-final size. Diamond grinding and polishing were then performedto achieve final size and surface finish for the components.

Example 2: Oxidation of Biocompatible Silicon Nitride Ceramic Components

The final components from Example 1 were subjected to hydrothermaloxidation using a steam autoclave at a pressure of 2 atm and atemperature of 121° C. for 24, 48, or 72 hours.

To determine the extent of the oxidation reaction, x-ray photoelectronspectroscopy was conducted on the oxidized components following 0 (FIG.2A and FIG. 2E), 24 (FIG. 2B and FIG. 2F), 48 (FIG. 2C and FIG. 2G), and72 (FIG. 2D and FIG. 2H) hours of exposure to the hydrothermal oxidationprocess. Further, the x-ray photoelectron spectroscopy analyzed thedeconvolution of the O1s and Si₂P bands. The results of thedeconvolution of the O1s band (FIG. 2E, FIG. 2F, FIG. 2G, and FIG. 2H)demonstrated a reduction of near-surface N—Si—O—Si bonds in favor ofO—Si—O—Si bonds. The results of the deconvolution of the Si₂P band (FIG.2E, FIG. 2F, FIG. 2G, and FIG. 2H) demonstrated a reduction of surfaceN—Si—N bonds in favor of N—Si—O and O—Si—O bonds. Both the deconvolutionof the O1s band (FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D) and Si₂P band(FIG. 2E, FIG. 2F, FIG. 2G, and FIG. 2H) indicated an increase inoxidation of the Si₃N₄ surface.

The statistical significance associated with these chemical bond changesis shown in FIG. 3A and FIG. 3B. The O1s band shows a reduction ofO—Si—N bonds in favor of O—Si—O bonds (FIG. 3A). The Si₂P band shows areduction in N—Si—N bonds in favor of O—Si—O bonds (FIG. 3B). These dataindicate that increasing exposure to the hydrothermal environment slowlyconverts Si₃N₄ from a mixed nitride-oxide surface to predominately anoxide condition. This is demonstrated by the microstructural photosprovided in FIG. 4A and FIG. 48. They show a pristine polished sampleprior to hydrothermal treatment (FIG. 4A). The pristine surface hasperiodic pits and defects that are filled with a silica (i.e., SiO₂)glass after its hydrothermal oxidation treatment (FIG. 48). Withoutbeing bound by theory, it is thought that engineering of this uniquesurface chemistry enables Si₃N₄ to serve as a superior articulationmember in total joint arthroplasty prostheses.

Example 3: Wear Testing

Femoral heads prepared as described in Examples 1 and 2 and femoralheads prepared with BIOLOX delta (zirconia-toughened alumina) weresubjected to wear testing using a hip joint simulator. Specifically, theacetabular cups were subjected to hydrothermal oxidation treatment for72 hours at 121° C. Briefly, the acetabular cups were weighted andpre-soaked in a bath comprising bovine serum to achieve a steady levelof fluid sorption (as recommended in ISO 14242/2). After 50 hours ofsoaking, all acetabular cups were cleaned and re-weighted. Thisprocedure was repeated until the incremental change of the acetabularcups over 24 hours was less than 10% of the previous cumulative masschange (as part ISO 14242—Part 2).

The acetabular cups were coupled to femoral heads and tested on a12-station hip joint simulator using a lubricant (25% sterile calf serum(Sigma Aldrich, St. Louis, Mo.) balanced with deionized water, 0.2%sodium azide, and 20 mmol/dm³ ethylenediaminetetraacetic acid (EDTA)).After every 400,000 cycles in the hip joint simulator, the weight lossof the acetabular cups was accessed. At each weight-stop the acetabularcups were removed and cleaned using a dedicated detergent, i.e., Clean65, at 40° C. for 15 minutes in an ultrasound washer. After rinsing, theacetabular cups were cleaned in an ultrasound washer comprisingdeionized water for an additional 15 minutes. The acetabular cups wereinitially dried using nitrogen and then placed under vacuum (0.1 bar)for 40 minutes to complete the drying. Weight loss was measured using amicrobalance. Each acetabular cup was weighted three times and theaverage was computed.

The weight loss vs. the number of cycles for the acetabular cups coupledwith the femoral heads is shown in FIG. 5. The Femoral heads prepared asdescribed in Examples 1 and 2 (labeled MC²® AMEDICA Si₃N₄ in FIG. 5) hada lower average mass loss (0.46 mg/Mc) than the average mass loss of theZTA BIOLOX® delta (0.55 mg/Mc).

Example 4: Static Hydrothermal Exposure

Femoral heads, prepared as described in Examples 1 and 2 and BIOLOXdelta were subjected to wear testing using a hip joint simulator in asimilar fashion to Example 3. However, the femoral heads werearticulated against E1 (a vitamin E infused polyethylene).

The results show the differences in the crystallinity and thecorresponding oxidation indices for E1 at the sliding surface (i.e.,z=0) (FIG. 6A) and at a depth of 200 μm (FIG. 6B) for both thenon-wear-(NWZ) and main-wear-zones (MWZ) of the liner. The Si₃N₄ wasremarkably effective in reducing the oxidation of the liner at thesurface (i.e., negative crystallinity and oxidation indices) whereas theoxidation increased for the liners articulating against ZTA. At a depthof 200 μm, the changes in crystallinity and oxidation indices for theSi₃N₄ remained near zero. Conversely, the liners articulating againstZTA showed marked increases in both parameters.

Example 5: Homeostatic Conditions

A block of silicon nitride ceramic as prepared in Example 1 was polishedand then embedded in an acidic gel. A pH microscope (SCHEM-110; Horiba,Kyoto, Japan) capable of measuring and mapping local pH values at thesurface of solids with high spatial resolution. In performing the pHmapping experiment, Si₃N₄ samples were fully embedded into an acidic gelconsisting of artificial saliva, KCl, and agar. The pH-imaging sensorconsisted of a flat semiconductor plate with a total sensing area of2.5×2.5 cm2. The highest spatial resolution and the pH sensitivity ofthe sensor were 100 μm and 0.1 pH, respectively. The microscope wasequipped with a light addressable potentiometric sensor, capable ofdetecting protons within the electrolyte. A light beam was directed fromthe back of the sensor with a bias voltage applied between theelectrolyte and the back. Since characterization of the AC photocurrent,which was induced by the modulated illumination from the back of thesensor, depended on the amount of protons at the sensor surface, the pHvalue was determined to a high degree of precision by measuring thelocal value of electric current. The detected current signals were thenconverted into a color scale, with each pixel correlated to the pH imageusing image analysis software (Image Pro Plus, Media Cybernetics, MD,USA). This generated a visual pH map around the embedded Si₃N₄ samples.After embedding the test pieces, pH maps were obtained at various timeintervals up to 45 min duration.

By using a pH microscope, a change in the acidity level next to theimplant was noted over a period of about 45 minutes.

Si₃N₄ surfaces are effective in altering the local pH due to theirslight dissolution and elution behavior (i.e., refer to the reactionsdescribed previously). The key results are shown in FIG. 7. Thisgraphical diagram shows that the pH surrounding the implant immediatelybegins to increase from its initial acidic value of 5.5 and reaches abasic condition at ˜8.5 over the 45 minute interval. The extent of thepH change can presumably be pre-engineered by altering the surfacechemistry of the implant (i.e., from a mixed nitride-oxide to an oxidesurface).

Example 6: Oxide Ceramic and Non-Oxide Ceramic Femoral Heads VersusUHMWPE Liners

Two types of oxide femoral heads (Al₂O₃, BIOLOX®forte andzirconia-toughened alumina, ZTA, BIOLOX®delta, CeramTec, GmbH,Plochingen, Germany) and one type of a non-oxide femoral head(MC²®Si₃N₄, Amedica Corporation, Salt Lake City, Utah, USA) were testedversus two advanced highly crosslinked ultra-high molecular weightpolyethylene liners (UHMWPE) including a sequentially irradiated andannealed material (X3, Stryker Orthopedics, Inc., Mahwah, N.J., USA) anda vitamin-E infused material (E1®, Zimmer Biomet, Warsaw, Ind., USA).

Four experiments in total were performed: (i) A preliminary hydrothermaltest in a water-vapor atmosphere as a function of time; (ii) A static,load-free, and short-term hydrothermal exposure of ceramic heads coupledwith UHMWPE liners with a wet interface; (iii) A frictionalreciprocating or “swing” test in lubricated environment; and, (iv) A hipsimulator test with bovine serum as a lubricant. Schematic diagrams ofthe testing procedures in (ii), (iii), and (iv) are represented in FIGS.8A, 8B, and 8C, respectively, with the main testing conditions given inthe insets of their respective diagrams.

In the static hydrothermal test of ceramic/UHMWPE couples (item (ii)above; FIG. 8A), six X3 UHMWPE liners equal in size and shape werecoupled to three types of Ø32 mm ceramic femoral heads (Al₂O₃, ZTA, andSi₃N₄). The liners had previously been gamma-ray irradiated with anaverage dose of 32 kGy. For comparison, six identical convex UHMWPEsamples were mated and tested against six spherical (concave) UHMWPEsections. The convex UHMWPE samples were not irradiated. Lightly clampedto assure a constant contact (i.e., 25 N), the couples were subjected toaccelerated an autoclave-aging test. All surfaces were dipped in purewater before being coupled and immediately placed into the autoclave at121° C. under adiabatic water-vapor pressure. The aging time waspurposely kept short at a fixed interval of 24 h, and all samples wereconcurrently run in the same experimental session. After the completionof the accelerated aging test, the samples were dried and cooled at arate of 100° C./h. The test was repeated three times, using two couplesfor each type of material during each experimental session.

The frictional swing test (item (iii) above; FIG. 8B) was conductedusing two types of Ø28 mm femoral heads (i.e., ZTA and Si₃N₄, n=3 each)coupled to X3 liners in a lubricated environment. The UHMWPE liners werepre-irradiated as described above. The wear testing apparatus consistedof a single station in plane reciprocating (or rocker motion) hipsimulator. The simulator consisted of a stepper motor with a reducinggear, which created a swing motion of ±20 at a frequency of 1 Hz with abrief (˜0.25 s) pause at +20° and −20°. The unit was placed in acompression-testing machine (600LX, Instron Corporation, Norwood, Mass.,USA) with a constant axial applied load of 1700 N through the entirecycle. The trunnion and liner were oriented at an angle of 33 toreplicate relevant physiologic loading. Wear testing was performed at anambient temperature (i.e., ˜25 C); the temperature was periodicallymonitored during testing. The basic composition of the lubricant used inthe test consisted of deionized water, two salts, (i.e., 8 mg/ml NaCland 2.68 mg/ml Na₂HPO₄.7H₂O) and two proteins (i.e., 11.1 mg/ml bovinealbumin and 5.1 mg/ml bovine γ-globulin). An addition of ˜0.29 mg/ml ofFeCl₃ to the basic lubricant was performed to replicate physiologicallyrelevant concentrations of Fe³⁺ ions (i.e., ˜100 mg/l) in the jointfluid. Each test sequence was carried out to 5×10⁵ cycles at 1 Hz.

In the hip simulator test, twelve E1®UHMWPE liners (six coupled to ZTAand six to Si₃N₄ femoral heads) were soaked in bovine calf serum for 4weeks prior to wear testing to compensate for weight changes due tofluid absorption in accordance with ISO 14242-2. As shown in FIG. 8C,wear tests were performed using an inverted-position type 12-station hipjoint simulator (Shore Western, Monrovia, Los Angeles, Calif.) inaccordance with ISO 14242-3. The articulating couples were subjected toa sinusoidal load with a peak of 2 kN and a frequency of 1.1 Hz inrotation. The weight loss of the liners was measured at 0.5 millioncycles (Mc) intervals using an analytical balance (Sartorius AG,Gottingen, Germany).

For comparison, two retrieved femoral heads, which had articulatedagainst polyethylene liners in vivo were also investigated. One was asecond generation monolithic Al₂O₃(Biolox®Forte, CeramTec, GmbH,Plochingen, Germany). It was retrieved after 26.3 y in vivo due to wearof the polyethylene liner. The second was the so-calledfourth-generation ZTA head (BIOLOX®delta, CeramTec, GmbH, Plochingen,Germany). It had been in vivo for 20 months articulating against a X3™(Stryker Orthopedics, Inc., Mahwah, N.J., USA) liner and was removed dueto a hip dislocation.

Example 7: Analytical Characterization

XPS analyses were performed on the surfaces of both ceramic femoralheads and UHMWPE samples described in Example 6 before and afterhydrothermal aging, static hydrothermal testing of ceramic/UHMWPEcouples, and frictional swing tests. A photoelectron spectrometer(JPS-9010 MC; JEOL Ltd., Tokyo, Japan) with an x-ray source ofmonochromatic MgKα (output 10 kV, 10 mA) was employed for theseanalyses. Surfaces of the samples were cleaned by Ar⁺ sputtering in thepre-chamber, while actual measurements were conducted in the vacuumchamber at around 2×10⁻⁷ Pa with an analyzer pass energy of 10 eV andvoltage step size of 0.1 eV. X-ray incidence and takeoff angles were setat 34° and 90°, respectively. The fraction of elemental oxygen wasdetermined by averaging three separate measurements on each of thetested UHMWPE liners at selected locations (e.g., wear zone and non-wearzone). Comparisons between the XPS outputs for ceramic and UHMWPEsamples served to assess the oxygen flow between the hip jointcounterparts. The sensitivity factors (in a %) used for the calculationof C, O, Si, and N were 4.079, 10.958, 2.387, and 7.039, respectively.

CL spectra were collected using a field-emission gun scanning electronmicroscope (FEG-SEM, SE-4300, Hitachi Co., Tokyo, Japan) equipped withan optical device. Upon electron irradiation with an accelerationvoltage of 5 kV (below the threshold for perturbation of thestoichiometric structure of the investigated ceramics), the emitted CLemission was collected with an ellipsoidal mirror connected through anoptical fiber bundle to a highly spectrally resolved monochromator(Triax 320, Jobin-Yvon/Horiba Group, Tokyo, Japan). A 150 g/mm gratingwas used throughout the experiments and a liquid nitrogen-cooled1024×256 pixels CCD camera collected the CL emissions. The resultingspectra were analyzed with the aid of commercially available software(LabSpec 4.02, Horiba/Jobin-Yvon, Kyoto, Japan). Mapping was performedusing a lateral step of 50 nm with an automatic collection of 1600measurement points per map. The CL probe size was on the order of 68×280nm in-depth and in-plane, respectively.

Raman assessments used a triple-monochromator (T-64000,Jobin-Ivon/Horiba Group, Kyoto, Japan) equipped with a charge-coupleddevice (CCD) detector. Automatic fitting algorithms for spectralde-convolution were obtained using a commercially availablecomputational package (LabSpec 4.2, Horiba/Jobin-Yvon, Kyoto, Japan).The in-depth spatial resolution of the Raman probe was confined to ˜6 μmby means of a 100× objective lens with a confocal pinhole (Ø100 μm)placed in the optical circuit. An automated sample stage was employed tocollect square maps (50×50 μm² with a square mesh of 5 μm steps) ofRaman spectra at different depths below the surface. Each UHMWPE samplewas characterized in three separate locations before and after theaccelerated aging test. Assuming that the oxidative phenomenon is theonly trigger for recrystallization, variations in the oxidation index(ΔOI) were calculated using a previously calibrated phenomenologicalequation.

FTIR spectroscopy (FT/IR-4000 Series, Jasco, Easton, Md., USA) was usedto monitor oxidation along the cross-section of the UHMWPE liners. Someof the tested liners were cut perpendicularly to the articulatingsurface, and a series of thin slices were obtained using a microtomedevice. The area of analysis was set at 100×100 μm². Spectra wererecorded at intervals of 100 μm parallel to the free surface of theliner. The spectra were always collected in transmission mode with aspectral resolution of 4 cm⁻¹. The oxidation index, OI, was computed asthe ratio of the area subtended by the infrared absorption bands ofpolyethylene located in the spectral interval 1650-1850 cm⁻¹ and thearea of the absorption bands located in the interval 1330-1396 cm⁻¹(i.e., emissions related to C—H bending). For a limited number ofsamples of both types of UHMWPE liners, the OI values obtained by FTIRwere compared with those obtained from Raman assessment of crystallinityvariation using previously calibrated algorithms for the same materials.The FTIR and Raman comparison confirmed previous findings using thesetesting procedures and validated the Raman algorithms for OI assessmentswithin a precision of ±5%.

The unpaired Students t-test was utilized for statistical analyses.Sample sizes are stipulated in each figure's insets. A p value <0.05 wasconsidered statistically significant and labeled with an asterisk.

Example 8: Surface Chemistry Changes Due to Hydrothermal Annealing

A preliminary procedure was designed to quantitatively assess chemicalchanges occurring in the oxide and non-oxide bioceramics due tohydrothermal exposure. This procedure utilized a combination of spectraldata acquired by XPS and CL spectroscopy.

FIGS. 9A, 9B, and 9C show average XPS spectra for Al2p inAl₂O₃(BIOLOX®forte), Al2p in ZTA (BIOLOX®delta), and Si2p in Si₃N₄(MC²),respectively, as received, and FIGS. 90, 9E and 9F show the sameceramics after 24 h adiabatic exposure in autoclave at 121° C.,respectively. The oxide spectra were deconvoluted into three Voigtiansub-band components: hydroxylated (O—Al—O—H) bonds, non-hydroxylated(O—Al—O) bonds, and O—Al—VO bonds representing the bond population atthe material surface. On the other hand, the non-oxide spectra includedthree sub-bands: one related to N—Si—N, and two additional ones fromdifferent types of Si—O bonds, namely N—Si—O and O—Si—O, which belong tothe bulk _(Si3N4) lattice and to a surface-formed silicon oxynitridelattice, respectively. A comparison between pristine and short-termautoclaved samples, indeed shows how quickly stoichiometric variationscommonly take place at the surface of both oxide and non-oxide ceramics.In both oxide samples, the fraction of O—Al—V_(O) bonds increased at theexpenses of both O—Al—O and O—Al—O—H bonds, while in the non-oxidesample both O—Si—O and N—Si—O types of bond underwent fractionalincrease at the expenses of the N—Si—N bond population.

FIGS. 10A-10F show XPS results collected as a function of exposure timein the autoclave (121° C.; 1 bar) by averaging n>6 measurementsperformed at n=6 different zones on the spherical surfaces of theceramic heads. In FIGS. 10A and 10B, results are shown for the Al2p edgeof the monolithic alumina and ZTA composite heads, respectively. The XPSspectra, fitted to the same Voigtian functions as shown in FIGS. 9A-9F,revealed homogeneous trends along with progressive reductions of O—Al—Obonds in favor of oxygen-vacancy O—Al—VO sites for both oxide ceramics(p<0.05). Closer inspection of the data showed a larger initial fractionof defective sites in the monolithic _(Al2O3) as compared to thecomposite ZTA. Also, more defects appeared in the Al₂O₃ with increasedautoclave time than in the ZTA (cf. FIGS. 10A and 10B). Nevertheless,oxygen gradually left the surfaces of both types of oxide heads althoughthis process occurred at different rates.

XPS data collected on the oxide components were then compared withvalues obtained under exactly the same experimental conditions for thenon-oxide Si₃N₄ heads. FIGS. 10C and 10D show the XPS trends detected atO1s and Si2p edges for Si₃N₄, respectively, as a function of autoclaveexposure. These latter data sets reveal the progressive fractionaldecrease in O—Si—N and N—Si—N bonds in favor of O—Si—O and N—Si—N sitesat the ceramic's surface (p<0.05). This indicates that surface nitrogenis gradually replaced by oxygen.

CL data for the two oxide-based ceramics are shown in FIGS. 11A-11D.FIGS. 11A and 11B show their morphological evolution of the CL spectraas a function of increasing autoclave time for femoral heads made ofAl₂O₃ and ZTA, respectively. Both materials showed an increasing opticalemission at around 325-330 nm, which corresponds to the formation ofoxygen vacancies. FIG. 11C compares the CL intensity of oxygen vacancyemissions from Al₂O₃ and ZTA over the entire investigated autoclavingtime. Consistent with the XPS data of FIGS. 10A-10D, the CL experimentsrevealed that the ZTA composite contained a lower initial amount ofoxygen vacancies and a milder increase of their population withautoclaving time as compared to monolithic Al₂O₃. These differences arelikely due to the presence of zirconia phase which reduced the arealfraction of oxygen-emitting alumina by ˜17 vol %. Additionally, thepresence of Cr³⁺ (i.e., a dopant intentionally added to substitute forAl³⁺) delays dehydroxylation due to its higher energy hydrogen-bondingas compared to Al³⁺. Note that the geometry of the electron probe inboth XPS and CL is similarly shallow (i.e., nanometer depth) whichsuggests that results from these two methods are comparable. FIG. 11Dlinks drifts in stoichiometry by XPS to increases in CL intensities forboth the Al₂O₃ and ZTA heads. These plots provide semi-quantitative datafor oxygen-vacancies formed in vitro at the surfaces of these twoceramics.

Similar CL experiments were conducted on the surfaces of Si₃N₄ heads asa function of autoclaving time (not shown). The propensity for oxygen toreplace nitrogen was reflected by an increased intensity of a CL band at˜650 nm which belongs to oxygen-excess sites (i.e., non-bridging oxygenhole centers) typical of silica glass.

FIGS. 10A-10D and 11A-11D reveal opposite scenarios for oxide andnon-oxide ceramics. Adsorption of molecular water plays the role of asolvent for the oxide ceramics with free oxygen flowing away from theirsurfaces, whereas it is an oxidant for Si₃N₄ and therefore oxygen flowstowards its surface. Water molecules possess different strengths uponhydrogen-bonding to the oxide and non-oxide ceramic surfaces (i.e.,aluminols and silanols for Al₂O₃-based and Si₃N₄ ceramics,respectively). Strong bonds result from H-bond acceptors when silanolsform and from H-bond donors at interfacial aluminols; whereas, weakbonds form from H-bond donors and acceptors at the surfaces of Si₃N₄ andAl₂O₃, respectively.

Example 9: Static Hydrothermal Test on Ceramic/Polyethylene Couples

The impact of oxygen movement on the crystallization and oxidation ofthe polyethylene liners when coupled to various ceramic femoral headswas initially examined using static hydrothermal-activated tests undernear zero loads. Data in this Example validate preliminary Raman/FT-IRcharacterizations of the crystallinity and oxidation of X3 highlycrosslinked polyethylene liners. Specifically, the aim of this Examplewas to confirm previous data using new experiments on the same brand ofadvanced polyethylene by adding XPS analyses of the polyethylenesurfaces to the prior Raman and FTIR characterizations. XPS analyses onthe ceramic surfaces were also performed, but they did not tangiblydiffer from the hydrothermal tests described in Example 8. Accordingly,FIGS. 10A-10D represent the results of the static hydrothermal testingof these ceramics when coupled to UHMWPE liners.

FIG. 12A shows crystallinity, Δc₀, and oxidation index variations, ΔOI₀,at the surfaces of the X3™ polyethylene liners with respect to theirpristine values. Polyethylene versus polyethylene couples (i.e., X3™ vs.X3™) with the same geometrical configuration as the ceramic versuspolyethylene couples were used as positive controls. The null hypothesiswas that all of the tested ceramics (if completely bioinert) wouldinduce the same variations in Δc₀ and ΔOI₀ as the all-poly-ethylenecouples. FIG. 12B summarizes the XPS results collected at thepolyethylene surface for each of the investigated couples.

Note that the data presented in FIGS. 12A and 12B dearly diverge fromthe null hypothesis. In the couples containing the oxide ceramics, asignificant increase in surface crystallization (˜55%) and oxidation(˜45%) was observed. The results were statistical valid when compared tothe positive control (polyethylene vs. polyethylene couples) while thedifference between the two oxide-containing couples was not significant.The liners coupled to the Si₃N₄ heads experienced ˜30% lower increase intheir oxidation indices than liners coupled to the oxide ceramics; andthey were only ˜14% higher than the control couples. The XPS data at theliner surfaces were consistent with vibrational data. They showed thehighest amount of oxygen at the surfaces of liners coupled to ceramicoxide heads (i.e., about twice the amount detected in theall-polyethylene couples), with no statistically significant differencebetween liners coupled to Al₂O₃ or ZTA. The oxygen content detected byXPS at the surfaces of the liners coupled to Si₃N₄ was only slightlyhigher (with no statistical relevance) than values detected at thesurface of the control couples. Traces of N and Si were found by XPS onthe surface of all tested liners; this was presumably due to annealingand polishing of the UHMWPE components, respectively, during theirmanufacture.

Assuming that the environmental loading on all of the samples was bothgeometrically and thermodynamically identical, it follows that theincrease in polyethylene oxidation for the oxide ceramic couples (ascompared to the controls) arises from oxygen emissions from the ceramicsurfaces. This hypothesis is consistent with the XPS data for theseliners (cf., FIGS. 10A and 10B and FIG. 12B). After 24 hours of exposurein the hydro-thermal environment, fractional increases of the O—Al-Vobonds in the oxide ceramics (˜50%) were nearly equal to the fractionalincreases in oxygen bonds detected at the surfaces of the polyethyleneliners.

In an attempt to quantify the potential protective action of the Si₃N₄head in preventing oxidation of the UHMWPE liner, an X3™ lineridentically exposed to the hydrothermal test conditions was subsequentlyspectroscopically characterized (n=3). This additional sample isreferred to as the “free” polyethylene. The Δc₀ and ΔOI₀ values for thissample were between the polyethylene control couple and the polyethyleneversus Si₃N₄ couple with no statistically significant differences withrespect to the two couples. Regarding the oxygen content detected by XPSat the surface of the “free” sample (FIG. 12B), it was slightly higherthan at the surface of the liner coupled with Si₃N₄; but this differencewas not statistically relevant.

In summary, non-oxide ceramics dearly proved to be more friendlycounterparts in delaying UHMWPE oxidation than the oxide ceramics inthis specific static hydrothermal test. Although the oxygencontamination by oxide ceramics was clearly quantified, any protectiveeffect by non-oxide ceramics in counteracting the degradation of UHMWPEliners needs to be assessed in longer-term hydrothermal experiments.

Example 10: Frictional Swing Test on Ceramic/Polyethylene Couples

An additional set of experiments was conceived based on frictionalinteractions between the two lubricated components of the couple underswing kinetics but left aside hydrothermal activation. The purpose ofthese tests was to determine the impact of different femoral headmaterials on the oxidation of UHMWPE (i.e., X3™) using frictionalsliding under a moderate load. FIGS. 13A and 13B show typical Al2p XPSspectra from ZTA femoral heads before and after this frictional swingtest for 5×10⁵ cycles at 1 Hz with a 1700 N load under lubricatedconditions, respectively. This frictional test induced significantalterations of the XPS spectrum at the surface of the oxide compositedemonstrating a drift in off-stoichiometry towards anoxygen-vacancy-rich environment. A quantitative plot of the variationsobserved in the Al2p spectra is given in FIG. 13C. This plot reveals a˜28% decrease of the O—Al—O bond population in favor of a nearlyequivalent increase of O—Al—VO bonds. The O1s edge consistently showed adecrease of Al—O—Al—O population in favor of Al—O—Al—VO while confirmingsurface de-hydroxylation with a significant reduction in the populationof Al—O—H bonds (FIG. 13D). On the one hand, the Zr3d edge of the ZTAsurface (also shown in FIG. 13D) revealed an invariant fraction ofZr—O—H and an increase in Zr—O—Zr bonds. This observation was consistentwith the fact that dehydroxylation hardly occurs in ZrO₂ ceramics due toa much stronger O—H bond as compared to the O—H bond at the surface ofAl₂O₃. On the other hand, its occurrence is a consequence of free oxygenfrom the tribolayer filling pre-existing vacancies in the metastabletetragonal (Y-doped) zirconia lattice, which in turn induces spontaneousphase transformation into the monoclinic polymorph.

FIGS. 14A and 148 represent typical N1s XPS spectra from the Si₃N₄femoral heads before and after the frictional swing test, respectively.In these cases, prolonged frictional exposure induced dramaticoff-stoichiometry at their surfaces with a decrease of ˜27% in Si—N—Si—Nbonds in favor of a nearly equivalent increase of Si—N—Si—O bonds, whilethe population of the Si—N—H bonds remained unaltered (cf., FIG. 14C).XPS data at the O1s edge confirmed the trend observed at the N1s edge. Areduction in N—Si—N bonds was also observed at the Si2p edge (cf., FIG.14D). The XPS data provided in FIGS. 13A-13D and FIGS. 14A-14Ddemonstrated opposite trends for oxygen chemistry at the surfaces of theZTA and Si₃N₄ ceramics due to their frictional loading against UHMWPEliners. The former material released oxygen from its surface (i.e., anincrease of Al—O—Al—VO bonds), while the latter scavenged oxygen (i.e.,an increase of Si—N—Si—O bonds).

In order to determine the effect this opposite movement of oxygen had onthe UHMWPE liners, their vibrational behavior was monitored as afunction of the number of swing cycles, n_(c). FIGS. 15A and 15B showcrystallinity and variations in oxidation indices as a function of n_(c)for liners coupled to ZTA and silicon nitride (data are from non-wearzones, NWZ, and main-wear zones, MWZ), respectively. The results ofFIGS. 15A-15B reveal that frictional contact increased surfacecrystallinity and oxidation indices for both NWZ and MWZ locationsindependent of whether the liners were coupled to oxide or non-oxideceramic heads. However, the UHMWPE degradation was significantly greaterin liners coupled to the ZTA heads, especially in the NWZ (i.e., ΔOI−1.2vs. 0.4 after 5×10⁵ cycles) In the MWZ, the average ΔOI for linerscoupled to the Si₃N₄ heads was the same as the NWZ (i.e., −0.4), whilethe liners coupled to ZTA was −0.7 lower than the NWZ. Note that thetrend in ΔOI vs. n_(c) in the MWZ tended to saturate for liners coupledto Si₃N₄, while it exponentially increased in the NWZ for liners coupledto both ZTA and Si₃N₄ ceramics. Accordingly, there are likely competingeffects in the MWZ between material removal from the liner due tofrictional wear and the rate of crystallization and oxidation of theUHMWPE's surfaces. The latter rate appears faster than the former.Consequently, the ΔOI continuously increased with n_(c). This appearedto be the case for the NWZ in which the material removal rate wasessentially zero. Conversely, a slower oxidation rate in comparison tofrictional material loss led to saturation of the ΔOI vs. n_(c) for theMWZ.

Based on the removal of the UHMWPE's machining marks and gravimetricanalyses, wear rates for both types of couples were similar (cf., lasermicroscopy results of FIG. 16A and weight loss values of −0.9 mg,respectively). FIG. 16B provides a comparison of XPS data collected atthe surface of the liners in both the MWZ and NWZ at n_(c)=5×10⁵. Thenumber of oxygen bonds at the NWZ surfaces of liners coupled to the ZTAheads was the highest in this set of experiments (−15 at %) and twofoldhigher than liners coupled to the Si₃N₄ heads. While the level of lineroxidation for Si₃N₄ couples was conspicuously the same for NWZ and MWZ,the ZTA couples showed higher oxidation in the NWZ as compared to MWZ(i.e., −15 vs. 12.5 at %). This suggests that the oxidation rate for theliners coupled to ZTA was faster than the corresponding material removalrate. This level of oxidation was definitely a preponderant phenomenonfor the ZTA couples, reaching OI values as high as −1.2 in the NWZ.These frictional swing-test experiments demonstrated that the oxidationof the UHMWPE liners, particularly those coupled to the ZTA heads, waspredominantly due to a chemical reaction rather than to mechanicalaction.

Example 11: Hip-Simulator Test of Ceramic/Polyethylene Couples

The crystallinity and oxidation of vitamin-E doped UHMWPE liners coupledto either ZTA or Si₃N₄ heads were evaluated after 5-million-cycles in astandard hip simulator test. This is part of an ongoing 12-million-cyclestudy aimed at evaluating the suitability of Si₃N₄ as an alternativeceramic bearing material. While anti-oxidant vitamin-E has demonstratedits ability to delay liner oxidation during in vitro experiments, thepurpose of these spectroscopic tests was to determine if the coupling ofvitamin-E doped UHMWPE liners to non-oxide ceramic heads could also leadto tangible advantages in terms of additional retardation of lineroxidation.

Both types of wear couples showed good performance. Average polyethyleneliner wear rates were 0.55 and 0.46 mg per million cycles for the ZTAand Si₃N₄ couples, respectively. FIGS. 17A and 17B comparecrystallinity, Δc, and oxidation index, ΔOI, variations for the ZTA andSi₃N₄ couples at the liner's surface and at a depth of 200 μm,respectively. Similar to the frictional swing test, the UHMWPE linerscoupled to ZTA had larger increases in both the amount ofcrystallization and the level of oxidation when compared to linerscoupled to Si₃N₄. The microstructural degradation of the UHMWPE was morepronounced at the surface than in the depth of the ZTA coupled liners.Conversely, no crystallization was apparent for the liners coupled toSi₃N₄ at either of the investigated depths. This was accompanied byessentially no change in the liners oxidation index (i.e., ΔOI˜0). Infact, a slight increase in amorphization was noted for the linersarticulated against Si₃N₄(FIG. 17A). Although a direct comparisonbetween the two types of UHMWPE liners (i.e., X3 v. E1®) has yet to bemade, it appears that the amount of surface oxidation associated withthe E1® liners was about one order of magnitude lower than the X3 inspite of the fact that the E1® liners had ˜10 times the number oftesting cycles. Nevertheless, an increase in the oxidation index for theE1® liners coupled to ZTA heads was a tangible result of this Example.With 5 million cycles being kinematically equivalent to about 2.5 yearsin vivo, it appears that addition of vitamin-E does not completelyeliminate liner oxidation in artificial hip joints coupled to oxideceramics.

Example 12: Retrieval Analyses

This Example provides an assessment of surface off-stoichiometry due tothe depletion of oxygen in oxide ceramic femoral heads retrieved fromhuman patients. These in vivo results are contrasted to the in vitroexperiments discussed in earlier Examples. Two retrieval cases arepresented as typical examples of both monolithic Al₂O₃ and ZTA heads.Conversely, Si₃N₄ is a new material and has not been cleared for use intotal hip arthroplasty; therefore retrievals are not yet available.

FIG. 18A shows a photograph of a femoral head from an earlier generationof monolithic Al₂O₃ articulating against a polyethylene liner for 26.3years in vivo. Scanning electron micrographs of its MWZ and NWZ surfaces(FIGS. 18B and 18C, respectively) revealed a relatively coarse granularstructure typical of early grades of biomedical alumina, with an averagegrain size ranging between 3 and 6 μm. Although grain boundaries weredearly visible—probably due to chemical etching in the acidic jointenvironment—no significant surface damage was observed in both the MWZand NWZ. This result is consistent with long-term articulation against asoft polyethylene counterpart. Cathodoluminescence emissions from oxygenvacancies (FIG. 18D) increased by ˜250% with respect to pristine aluminaheads; these were matched by a ˜153% increase in O—Al—VO bonds detectedby XPS (Al2p edge, FIG. 18E). Conversely, the number of Al—O—Al andO—Al—O—H bonds decreased by 34% and 26%, respectively.

The photograph in FIG. 19A is that of a ZTA femoral head whicharticulated only for 20 months in vivo against an X3 liner, (i.e., thesame liners tested in vitro, cf. FIGS. 12, 15, and 16). Metalcontamination is visible on the head's surface due to severaldislocation events, which preceded its revision surgery. The finemicrostructure, imaged in the MWZ and NWZ by scanning electronmicroscopy (FIGS. 19B and 19C), respectively, consisted of Al₂O₃ (darkercolor) and ZrO₂ (whitish) grains with average sizes of ˜1 and 0.4 μm,respectively. Both zones indicated that the head was essentiallyundamaged because typical machining marks from its manufacturing processwere evident on its surface. The MWZ and NWZ emitted similarCL-intensities from oxygen vacancies, both of which were higher by ˜450%compared to pristine components (FIG. 19D). XPS (Al2p) detected a ˜213%increase in O—Al—Vo bonds (FIG. 19E) accompanied by a ˜108% decrease inthe atomic fraction of Al—O—Al bonds. However, unlike the monolithicalumina head described in FIGS. 18A-18E, the population of O—Al—O—Hincreased by ˜288% with respect to pristine ZTA heads; this could berelated to the presence of the Cr³⁺ dopant in the alumina lattice whichhas a stronger hydrogen bond.

In substance, both CL and XPS independently detected a significantlyhigher population of oxygen vacancies at the surface of both long- andshort-term femoral head retrievals made of alumina-based ceramics.Moreover, the off-stoichiometry observed on the retrievals' surfaceswere significantly higher than those induced in the same materialsduring in vitro experiments. Characterization of these retrievalsconfirmed that a non-negligible amount of oxygen was released into thetribolayer from their surfaces. Indeed, the amount of oxygen releasedeven from the short-term retrieval is striking. The combination of anacidic hydrothermal environment, which is typical of synovial fluid inosteoarthritic patients, along with stronger frictional forces thanthose applied in the in vitro experiments was likely responsible for themarked trend in its observed oxygen deficiency.

The disclosures shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, especially inmatters of shape, size and arrangement of the parts within theprinciples of the present disclosure to the full extent indicated by thebroad general meaning of the terms used in the attached claims. It willtherefore be appreciated that the examples described above may bemodified within the scope of the appended claims.

What is claimed is:
 1. A silicon oxynitride material, wherein thesilicon oxynitride material has improved wear performance, and whereinthe silicon oxynitride material is prepared by a process comprising:forming a silicon nitride material block; and oxidizing the siliconnitride material block.
 2. The product of the process of claim 1,wherein forming the silicon nitride material block comprises: preparinga slurry comprising silicon, oxygen, and nitrogen, and furthercomprising at least one of yttrium oxide and aluminum oxide; milling theslurry; and drying the slurry to obtain a dried slurry.
 3. The productof the process of claim 1, wherein the silicon oxynitride materialcomprises a first crystalline phase and a first amorphous phase.
 4. Theproduct of the process of claim 1, wherein oxidizing the silicon nitridematerial block is performed using hydrothermal oxidation.
 5. The productof the process of claim 4, wherein the hydrothermal oxidation isperformed in a steam autoclave.
 6. The product of the process of claim4, wherein the hydrothermal oxidation is conducted at a pressure rangingfrom about 1 atmosphere to about 250 atmospheres.
 7. The product of theprocess of claim 4, wherein the hydrothermal oxidation is conducted at apressure of about 2 atmospheres.
 8. The product of the process of claim4, wherein the hydrothermal oxidation is conducted at a temperatureranging from about 100° C. to about 150° C.
 9. The product of theprocess of claim 4, wherein the hydrothermal oxidation is conducted at atemperature ranging from about 120° C. to about 135° C.
 10. The productof the process of claim 4, wherein the hydrothermal oxidation isconducted at a temperature of about 132° C.
 11. The product of theprocess of claim 4, wherein the hydrothermal oxidation is conducted fora duration ranging from about 50 to about 200 hours.
 12. The product ofthe process of claim 4, wherein the hydrothermal oxidation is conductedfor a duration ranging from about 70 to about 150 hours.
 13. The productof the process of claim 4, wherein the hydrothermal oxidation isconducted for a duration of about 72 hours.
 14. The product of theprocess of claim 1, wherein the silicon nitride material block is anarticulation component of a prosthetic joint.
 15. The product of theprocess of claim 14, wherein the articulation component is a femoralhead.
 16. The product of the process of claim 14, wherein the improvedwear performance increases the longevity of the prosthetic joint greaterthan 15 years.
 17. The product of the process of claim 14, wherein thesilicon nitride material has a surface chemistry that protects a countersurface of the articulation component from oxidation.
 18. The product ofthe process of claim 17, wherein the counter surface is an acetabularpolyethylene cup.